The applicant's patent application DE 10 2015 211 298.5, which does not constitute a prior publication, discloses a hydraulic system of an automatic transmission having a hydraulically actuable parking lock unit. A parking lock valve is configured with multiple valve pockets formed in the region of a valve housing, by which an actuation pressure of the parking lock unit, which actuation pressure prevails in a manner dependent on a supply pressure, can be applied to said parking lock unit. The valve pockets can be placed in operative connection with one another, or separated from one another, by a valve slide which is longitudinally displaceable in the valve housing. The valve slide is spring-loaded in the direction of a first axial position which corresponds to a first operating state of the parking lock valve and in which, in turn, an actuation pressure can be applied to the parking lock unit at which the parking lock transitions into the engaged operating state. In the region of a control surface of the valve slide, a pressure signal which acts in the direction of a second axial position of the valve slide, which corresponds to a second operating state of the parking lock valve, can be introduced by applying an actuation pressure to the parking lock unit, which actuation pressure transfers a parking lock of the parking lock unit into the disengaged operating state.
If a fault arises in the transmission system during operation with the parking lock disengaged, which fault leads to the deactivation of the electrical supply of the transmission actuators, the automatic transmission switches to so-called hydraulic emergency operation. During hydraulic emergency operation the parking lock valve is held in the operating state which corresponds to the disengaged operating state of the parking lock, by the pressure in the primary system pressure circuit in the region of a control surface of the valve slide of the parking lock valve, which control surface corresponds to a differential surface between two facing face surfaces of two valve slide sections configured with different diameters, counter to the spring force, until the system pressure falls below a defined pressure threshold which is dependent on the size of the differential surface and on the spring force.
Said pressure threshold is configured for driving states in which a clutch gear logic is implemented without actuation of a shift element which is actuated by the pressure signal. In the absence of a pressure signal and in the presence of a simultaneously low torque to be transmitted via the automatic transmission, the hydraulic system is operated with a low system pressure level in order to improve an efficiency of the automatic transmission. This means that, for the least possible power consumption of the transmission system, it is sought to achieve the lowest possible system pressures in the consumption-relevant operating range. This however has the result that the self-holding pressure of the parking lock valve lies below the minimum system pressure level, because otherwise, the parking lock cannot, in the presence of low system pressures, be held any longer in the disengaged operating state by the system pressure.
Owing to the selection of the features of the characteristic curves of the actuators, the shift elements of the automatic transmission are, in emergency operation, actuated in unpressurized fashion, while the system pressure assumes its maximum value. In the event of a transition to hydraulic emergency operation, it is additionally the case that the electromechanical locking mechanism of the parking lock is deactivated, with the result that inadequate hydraulic actuation of the parking lock device during emergency operation results in an immediate actuation of the parking lock device in the direction of the engaged operating state.
To be able to briefly compensate undersupplied operating states of the hydraulic system during a normal operating state of the automatic transmission, and to be able to operate the automatic transmission with high spontaneity over a large operating range of a vehicle drivetrain, it is possible for the above-described primary system pressure circuit to be implemented with a hydraulic fluid volume accumulator known from DE 10 2013 209 932 A1. For this purpose, a hydraulic fluid volume is temporarily stored counter to a spring force of a spring device in the region of the hydraulic fluid volume accumulator, which hydraulic fluid volume can, in accordance with demand, be introduced into the line system of the hydraulic system downstream of a check valve device. During hydraulic emergency operation of the automatic transmission, the hydraulic fluid volume accumulator is fully charged by the system pressure set to a maximum, and is preloaded to its maximum pressure value.
If a vehicle in which the transmission is in hydraulic emergency operation is shut down, a transmission main pump driven by the drive machine conveys no further hydraulic fluid volume into the hydraulic system, as is known. The previously set maximum system pressure thereby collapses, until it reaches the maximum pressure level of the hydraulic fluid volume accumulator. From this point in time onward, all actuated pressures in the hydraulic system are determined by the discharge pressure of the hydraulic fluid volume accumulator, and as a result both the system pressure and a pressure set with a falling characteristic curve in the region of a system pressure regulator assume the same pressure level.
From said point in time onward the gradient with which the system pressure in the primary pressure circuit is dissipated is dependent only on the leakage of the primary pressure circuit, and is thus highly dependent on the present operating temperature of the transmission. In particular in the presence of low operating temperatures, the leakage volume flows of the primary pressure circuit are very small. Since the minimum discharge pressure of the hydraulic fluid volume accumulator is above the self-holding pressure threshold of the parking lock valve, the engagement of the parking lock after the shutdown of the vehicle is delayed, however, to an undesired extent.